1 00:00:15 --> 00:00:22 OK. So the thing that we're going to talk today about are 2 00:00:22 --> 00:00:30 macromolecules. And often these are polymers. 3 00:00:30 --> 00:00:37 Now, all macromolecules comprise of multiple carbons joined together in 4 00:00:37 --> 00:00:44 some way. But the real definition of a polymer is that there is some 5 00:00:44 --> 00:00:51 kind of monomer that is joined together in a repeated way to make 6 00:00:51 --> 00:00:59 some kind of multimer that forms the polymer. OK. 7 00:00:59 --> 00:01:03 Now, integral to understanding all of this is the notion of chemical 8 00:01:03 --> 00:01:07 bonds. And you're going to, in this week's Section you're going 9 00:01:07 --> 00:01:12 to be addressing, remembering, understanding what 10 00:01:12 --> 00:01:16 chemical bonds are. And here are some concepts that are 11 00:01:16 --> 00:01:21 essential that you understand for chemical bonds. 12 00:01:21 --> 00:01:25 Let me list them on the board. And then we'll go through them in 13 00:01:25 --> 00:01:30 some more detail. And then I'm going to move on. 14 00:01:30 --> 00:01:35 And you are going to deal with this more in Section, 15 00:01:35 --> 00:01:40 but this is really integral to understanding the rest of the course. 16 00:01:40 --> 00:01:46 So one of the terms you need to know is electronegativity. 17 00:01:46 --> 00:01:51 Anyone want to give me an electronegativity definition? 18 00:01:51 --> 00:01:57 I may be able to find another chocolate fish for an 19 00:01:57 --> 00:02:02 electronegativity definition. Where? A little hand. 20 00:02:02 --> 00:02:07 Yeah? Say it again. OK. Good. Yeah. How strongly an atom 21 00:02:07 --> 00:02:13 attracts electrons. This is good. You see, 22 00:02:13 --> 00:02:18 you get to practice catching and I get to practice throwing. 23 00:02:18 --> 00:02:23 So the relative affinity of a particular atom for electrons. 24 00:02:23 --> 00:02:29 OK? So that's one think you need to know. Valance. 25 00:02:29 --> 00:02:34 Super important. Valance. Yeah? 26 00:02:34 --> 00:02:40 How many electron slots available for connections. 27 00:02:40 --> 00:02:46 Try to rephrase. Yes, but rephrase it a bit. 28 00:02:46 --> 00:02:51 We're talking about chemical bonds. So? OK, how many bonds. I did not 29 00:02:51 --> 00:02:57 give that away. OK. How many bonds a molecule can, 30 00:02:57 --> 00:03:03 an atom can make. OK? OK. And then we get into the 31 00:03:03 --> 00:03:11 bonds per se. And I'm going to list them here. I'm going to list them 32 00:03:11 --> 00:03:19 in order from strong to weak. Covalent bonds. Ionic. I'll go 33 00:03:19 --> 00:03:29 through some slides in a moment. 34 00:03:29 --> 00:03:33 Hydrogen and something I'll call hydrophobic. OK. 35 00:03:33 --> 00:03:38 And covalent bonds are the strongest and these so-called 36 00:03:38 --> 00:03:42 hydrophobic bonds are the weakest bonds. Ones that we are 37 00:03:42 --> 00:03:47 particularly interested in, actually, we're interested in all of 38 00:03:47 --> 00:03:51 them, but I would say covalent, hydrogen and hydrophobic bonds are 39 00:03:51 --> 00:03:56 particularly important. And you will see when we talk about 40 00:03:56 --> 00:04:01 DNA, hydrogen bonds are very important. 41 00:04:01 --> 00:04:07 So let's go through some pictures. This is a table from your book, I'm 42 00:04:07 --> 00:04:13 not going to dwell on too much, but you really need to be familiar 43 00:04:13 --> 00:04:20 with this, you need to understand what is in this. 44 00:04:20 --> 00:04:26 And it will indicate for you what the nature of the different 45 00:04:26 --> 00:04:32 bonds are. OK. Covalent bonds, 46 00:04:32 --> 00:04:36 really, the two atoms become very closely joined together. 47 00:04:36 --> 00:04:40 And, again, you're going to deal with this. Let me just move onto, 48 00:04:40 --> 00:04:44 in your recitation. Let me deal with two things that I want to dwell 49 00:04:44 --> 00:04:49 on a little bit. The first is the notion of polar 50 00:04:49 --> 00:04:53 and nonpolar molecules. OK? So polar molecules have got 51 00:04:53 --> 00:04:57 differential charge distribution. All right? So we can add that 52 00:04:57 --> 00:05:02 actually to our list. If you want to go back and add 53 00:05:02 --> 00:05:07 something to your list, its part and parcel of the notion of 54 00:05:07 --> 00:05:11 electronegativity, but let's also write polar versus 55 00:05:11 --> 00:05:16 nonpolar molecules. In a polar molecule, 56 00:05:16 --> 00:05:21 the charges in the molecule are asymmetrically distributed. 57 00:05:21 --> 00:05:25 Whereas, in the nonpolar molecule the charges are uniformly 58 00:05:25 --> 00:05:30 distributed and there is no net asymmetry of charge in that 59 00:05:30 --> 00:05:35 particular molecule. And that will become important in a 60 00:05:35 --> 00:05:40 little bit. Here is one that, again, you will dwell on in Section 61 00:05:40 --> 00:05:44 but I will introduce now. Hydrogen bonds are between a 62 00:05:44 --> 00:05:49 hydrogen and an oxygen, and the hydrogen or something else, 63 00:05:49 --> 00:05:54 something that is more electronegative, 64 00:05:54 --> 00:05:59 the hydrogen, it's the hydrogen, this is not a covalent bond. 65 00:05:59 --> 00:06:05 And the hydrogen interaction is integral to the structure of DNA, 66 00:06:05 --> 00:06:11 as we will discuss in this lecture and when we talk about molecular 67 00:06:11 --> 00:06:17 biology. OK. So that is a very cursory introduction to something 68 00:06:17 --> 00:06:23 that you will be dwelling on more in Section this week. 69 00:06:23 --> 00:06:30 OK. So I want to talk now about polymers. 70 00:06:30 --> 00:06:36 And I want to talk how they form biologically. And I'm going to tell 71 00:06:36 --> 00:06:43 you about two types of reactions that you should understand. 72 00:06:43 --> 00:06:49 Condensation reaction which forms a polymer and a hydrolysis reaction 73 00:06:49 --> 00:07:06 which breaks a polymer. 74 00:07:06 --> 00:07:12 So let's look at a diagram here that I've taken from your book. 75 00:07:12 --> 00:07:18 Here are two monomers. And you can see there is a hydroxyl group and a 76 00:07:18 --> 00:07:24 hydrogen group and a hydrogen. And those two things react with one 77 00:07:24 --> 00:07:30 another and, with the use of energy, water is released, and the two 78 00:07:30 --> 00:07:34 monomers are joined to one another. OK? Again, you're going to have 79 00:07:34 --> 00:07:38 plenty of practice at this. Don't try to draw this diagram here. 80 00:07:38 --> 00:07:42 OK? You can, we'll deal with it in a little bit. 81 00:07:42 --> 00:07:46 Just look at it and understand what I'm saying at the time and it really 82 00:07:46 --> 00:07:50 is sufficient. So, here, you've got the exclusion 83 00:07:50 --> 00:07:54 of water, the use of energy to get two monomers joined up, 84 00:07:54 --> 00:07:58 and then the process continues and you get a polymer being built up of 85 00:07:58 --> 00:08:03 more and more monomers. Now, the converse of that is 86 00:08:03 --> 00:08:09 hydrolysis where you add water and also use energy to break up a 87 00:08:09 --> 00:08:15 polymer and release a monomer. So either hydrolysis reaction or a 88 00:08:15 --> 00:08:21 condensation reaction are energy requiring reactions. 89 00:08:21 --> 00:08:27 And this energy has to come from somewhere. And we will spend a 90 00:08:27 --> 00:08:34 lecture talking about where the energy comes from. 91 00:08:34 --> 00:08:38 So I want to draw the analogy, and it may seem hokey to you, but 92 00:08:38 --> 00:08:43 it's actually quite useful, a useful way for me to think about 93 00:08:43 --> 00:08:47 this, so I want to share it with you. I want to draw the analogy to the 94 00:08:47 --> 00:08:52 cell as a factory. And I want to begin by talking 95 00:08:52 --> 00:08:56 about the materials that the factory uses to synthesize whatever it is 96 00:08:56 --> 00:09:01 it's going to synthesize. And then as we move through the 97 00:09:01 --> 00:09:05 biochemistry section I'll talk about the machinery within the cell that 98 00:09:05 --> 00:09:10 makes the synthesis take place. So we're going to talk about the 99 00:09:10 --> 00:09:14 materials that are present in the cell. And particularly we're going 100 00:09:14 --> 00:09:19 to talk about four groups of macromolecules, 101 00:09:19 --> 00:09:23 the big four from which everything else is built and that really are 102 00:09:23 --> 00:09:28 pivotal for life. These are proteins. 103 00:09:28 --> 00:09:33 And, actually, let me not list them in that order. 104 00:09:33 --> 00:09:38 Let me list them in the order in which I'm going to talk about them. 105 00:09:38 --> 00:09:43 I'm going to talk about lipids, carbohydrates, 106 00:09:43 --> 00:09:49 nucleic acids. And I'll talk about those three today. 107 00:09:49 --> 00:09:54 And then next time I'll talk about proteins. So if you look at the dry 108 00:09:54 --> 00:10:00 mass of a cell it's very interesting. 109 00:10:00 --> 00:10:06 Lipids comprise about 5% of the dry mass, carbohydrates about 25%, 110 00:10:06 --> 00:10:13 nucleic acids 10%, and proteins about 55%. OK. 111 00:10:13 --> 00:10:19 So that's what we're going to be talking about, 112 00:10:19 --> 00:10:26 these four groups of macromolecules that make up the components 113 00:10:26 --> 00:10:33 of the cell. Now, two things that I want to point 114 00:10:33 --> 00:10:40 out as we talk about these macromolecules is that very often in 115 00:10:40 --> 00:10:47 biology the polymers have two properties that are very important. 116 00:10:47 --> 00:10:54 One, they have polarity. That means one end is different from the 117 00:10:54 --> 00:11:00 other end. OK? And the other thing that they very 118 00:11:00 --> 00:11:05 often have, not always but some classes in particular, 119 00:11:05 --> 00:11:10 is a linear order. OK? So that's, both of those properties, 120 00:11:10 --> 00:11:15 if you give this a moment's thought, you will see are very similar to 121 00:11:15 --> 00:11:20 sentences. There's a beginning and an end of a sentence, 122 00:11:20 --> 00:11:25 and within a sentence the words come in a particular order. 123 00:11:25 --> 00:11:30 And that gives you an information content. That gives you a way to 124 00:11:30 --> 00:11:35 carry out or to store information. So I'll point this out. 125 00:11:35 --> 00:11:40 This is particularly true for the proteins and the nucleic acids. 126 00:11:40 --> 00:11:45 It's also somewhat true for carbohydrates and less so for lipids. 127 00:11:45 --> 00:11:50 So these two properties of biological polymers are fascinating 128 00:11:50 --> 00:11:55 and they are crucial for life to proceed. All right. 129 00:11:55 --> 00:12:00 So let's move onto one of the big four. Lipids. 130 00:12:00 --> 00:12:05 Lipids. So lipids are a somewhat large and almost amorphous group of 131 00:12:05 --> 00:12:11 molecules in that I cannot give you a chemical formula, 132 00:12:11 --> 00:12:17 really, to say this is definitely a lipid and this is not a lipid. 133 00:12:17 --> 00:12:23 They have one property that is crucial for defining them as lipids, 134 00:12:23 --> 00:12:29 and this is that they are hydrophobic -- 135 00:12:29 --> 00:12:37 -- or nonpolar. 136 00:12:37 --> 00:12:41 And this property of being hydrophobic, so lipids in foods. 137 00:12:41 --> 00:12:45 What are the lipids just, you know, in your food pyramid? It's just to 138 00:12:45 --> 00:12:49 make sure we're all talking about the same stuff here. 139 00:12:49 --> 00:12:53 Fats and oils, right. Yes. And you can come and get a 140 00:12:53 --> 00:12:57 fish later. OK. So fats and oils. 141 00:12:57 --> 00:13:02 They're used for a couple of things. One of the things about fats and 142 00:13:02 --> 00:13:07 oils is that they store a lot of energy. So they serve as a 143 00:13:07 --> 00:13:12 long-term energy source for the cell. And the other thing they do really 144 00:13:12 --> 00:13:18 has to do with their insulation properties. And their insulation 145 00:13:18 --> 00:13:23 properties fall into multiple categories. The most important one 146 00:13:23 --> 00:13:28 we touched on last time when we talked about the components 147 00:13:28 --> 00:13:33 of a cell. We talked about the fact that the 148 00:13:33 --> 00:13:38 cell is a cell, and the organelles in the cell are 149 00:13:38 --> 00:13:43 organelles because they are membrane bound. And I threw out at you the 150 00:13:43 --> 00:13:48 term lipid bilayer, and I'll throw it out at you again 151 00:13:48 --> 00:13:53 in a moment. This is something made of lipids that insulates the cell 152 00:13:53 --> 00:13:58 from its environment or subcellular compartments from one another. 153 00:13:58 --> 00:14:04 So part of the insulation property are membranes. 154 00:14:04 --> 00:14:09 And, again, this term, I'll write on the board this time, 155 00:14:09 --> 00:14:15 lipid bilayer. And the other aspect of insulation or the more classical 156 00:14:15 --> 00:14:21 things you think about, you don't get cold because you've 157 00:14:21 --> 00:14:27 got a layer of fat under your skin, and that stops yourself from losing 158 00:14:27 --> 00:14:32 heat. So heat loss. And, for example, 159 00:14:32 --> 00:14:37 birds use lipids to waterproof their feathers so that they can stand in 160 00:14:37 --> 00:14:41 the rain and not get wet. OK. Another aspect that I'll throw 161 00:14:41 --> 00:14:46 out at you also here is that lipids also make things called hormones, 162 00:14:46 --> 00:14:51 which are substances that control various aspects of body function. 163 00:14:51 --> 00:14:55 I'll have a little more to say about them in a couple of minutes, 164 00:14:55 --> 00:15:00 and then will have quite a bit more to say about them when we talk about 165 00:15:00 --> 00:15:05 reproduction. So let me see what is next here. 166 00:15:05 --> 00:15:11 OK. So let's talk about some typical lipids and some examples of 167 00:15:11 --> 00:15:16 lipids. This is a diagram from your book, so you can go and look at it 168 00:15:16 --> 00:15:22 at your leisure, of a triglyceride. 169 00:15:22 --> 00:15:28 Now, what I want you to take from this is that, so triglycerides, 170 00:15:28 --> 00:15:34 tri meaning three. And the glyce- part is from glycerol, 171 00:15:34 --> 00:15:40 which is this molecule up here, onto which three long carbon chains 172 00:15:40 --> 00:15:47 are added. And these long carbon chains are called fatty acids. 173 00:15:47 --> 00:15:53 And that's important to know. So a triglycerides is glycerol onto which 174 00:15:53 --> 00:16:00 three fatty acid chains have been added. 175 00:16:00 --> 00:16:07 And it's the fatty acid chains that are hydrophobic. 176 00:16:07 --> 00:16:15 So triglycerides are a very common 177 00:16:15 --> 00:16:21 type of lipid. Now, after I just told you, 178 00:16:21 --> 00:16:28 ah, it's fixed. After I just told you that lipids were 179 00:16:28 --> 00:16:35 all hydrophobic. I'm now going to tell you that some 180 00:16:35 --> 00:16:42 lipids are amphipathic. And they're amphipathic, 181 00:16:42 --> 00:16:49 so amphipathic means that they have both polar plus nonpolar components. 182 00:16:49 --> 00:16:56 And one of the most important amphipathic molecules in the lipid 183 00:16:56 --> 00:17:02 field is a phospholipid. So this is a molecule that's got 184 00:17:02 --> 00:17:08 these fatty acid long carbon chains on one end and on the other end has 185 00:17:08 --> 00:17:14 got a very polar phosphate group. OK? And here's phosphate with 186 00:17:14 --> 00:17:20 something called choline added onto it. So this is phosphatidylcholine. 187 00:17:20 --> 00:17:26 And phospholipids, and I spelt amphipathic incorrectly. 188 00:17:26 --> 00:17:32 Phospholipids are essential for formation of the lipid bilayer 189 00:17:32 --> 00:17:38 membranes. And the reason they are is because of this dual charged and 190 00:17:38 --> 00:17:44 noncharged portion. So here is a diagram that I showed 191 00:17:44 --> 00:17:50 you last time that was not as well labeled. This is a diagram of a 192 00:17:50 --> 00:17:56 lipid bilayer, so one of the membranes that 193 00:17:56 --> 00:18:02 surrounds your cells and your organelles. 194 00:18:02 --> 00:18:07 Facing out onto the water of the cell are the hydrophilic heads of 195 00:18:07 --> 00:18:12 this phospholipids, the charged heads. And then facing 196 00:18:12 --> 00:18:17 one another are two rows, OK, of these hydrophobic fatty acid 197 00:18:17 --> 00:18:22 tails. So the bilayer is because you have one layer of these 198 00:18:22 --> 00:18:27 molecules with the hydrophilic heads facing the water, 199 00:18:27 --> 00:18:32 the hydrophobic tails pointing in. And those interact with another 200 00:18:32 --> 00:18:36 layer where the hydrophilic heads are pointing to the water and the 201 00:18:36 --> 00:18:40 hydrophobic tails are pointing inwards. The hydrophobic tails 202 00:18:40 --> 00:18:45 interact with one another and you get this very stable lipid bilayer. 203 00:18:45 --> 00:18:49 OK. It's very cool. That's how things work, and they work very 204 00:18:49 --> 00:18:53 easily. And you can get them to be synthesized in a test-tube very 205 00:18:53 --> 00:18:58 easily. Here is another one. Saturated versus unsaturated fats. 206 00:18:58 --> 00:19:08 Unsat, OK, as an unsaturated. 207 00:19:08 --> 00:19:14 In saturated fats, the fatty acid side chain contains no double bounds. 208 00:19:14 --> 00:19:20 OK? So it saturated four hydrogens. In unsaturated fats, there are some 209 00:19:20 --> 00:19:26 double bonds that would allow one to add more hydrogens if they 210 00:19:26 --> 00:19:32 were broken. OK? So saturated versus unsaturated fats. 211 00:19:32 --> 00:19:38 So saturated fats are the things you find in butter and in hard fats. 212 00:19:38 --> 00:19:44 And they're supposed to be bad for you. OK? So saturated fats have 213 00:19:44 --> 00:19:50 been implicated in raising something called low density lipoproteins 214 00:19:50 --> 00:19:56 which are involved in various aspects of cell function that are 215 00:19:56 --> 00:20:03 believed to lead to heart disease. But it turns out that there's more 216 00:20:03 --> 00:20:09 to it than that. So if you've ever been really bored 217 00:20:09 --> 00:20:15 and you've read the ingredients on the box of cookies that you've eaten, 218 00:20:15 --> 00:20:21 you may see something that says partially hydrogenated vegetable oil. 219 00:20:21 --> 00:20:27 Yeah? OK. Now, your vegetable oil is rather 220 00:20:27 --> 00:20:32 unsaturated. OK? It's got a lot of double bonds. 221 00:20:32 --> 00:20:37 And one of the reasons that it is liquid is because these double bonds 222 00:20:37 --> 00:20:42 are there and you don't get the chains of the lipid packing very 223 00:20:42 --> 00:20:46 tightly and it's a liquid. But you can take your vegetable oil 224 00:20:46 --> 00:20:51 with lots of unsaturated bonds and you can partially saturate it. 225 00:20:51 --> 00:20:56 And you can get out of that two kinds of partially saturated fat, 226 00:20:56 --> 00:21:01 partially hydrogenated fat. You can get this stuff called 227 00:21:01 --> 00:21:05 transunsaturated fat. Now, if you look at this diagram, 228 00:21:05 --> 00:21:10 you will see these fatty acid side chains are in nice order here. 229 00:21:10 --> 00:21:14 OK? And they are because the hydrogens that are across from the 230 00:21:14 --> 00:21:18 double bonds are intrans from one another. They're kind of diagonally 231 00:21:18 --> 00:21:23 opposite from one another. And what that does is to allow 232 00:21:23 --> 00:21:27 these side chains to pack very tightly, and you get a very 233 00:21:27 --> 00:21:32 hard fat that way. Somehow this partially hydrogenated 234 00:21:32 --> 00:21:36 stuff is very bad for you. It raises your LDL, it reduces this 235 00:21:36 --> 00:21:40 stuff called high density lipoprotein, which is good for you, 236 00:21:40 --> 00:21:44 and it increases your heart disease risk. Interestingly, 237 00:21:44 --> 00:21:48 you do not find these types of molecules in nature. 238 00:21:48 --> 00:21:52 You only find them when you hydrogenate your vegetable oil and 239 00:21:52 --> 00:21:56 make your margarine or make the stuff that McDonald's cooks its 240 00:21:56 --> 00:22:00 French fries in, OK, and its chicken nuggets. 241 00:22:00 --> 00:22:04 So a couple of years ago there was so much issue about this that 242 00:22:04 --> 00:22:09 McDonald's pledged to find a different vegetable oil in which to 243 00:22:09 --> 00:22:13 fry its fries. And it's still searching, 244 00:22:13 --> 00:22:18 apparently, because there's some issue with taste being compromised 245 00:22:18 --> 00:22:22 as you go to healthier fats. OK. Now, in contrast to the 246 00:22:22 --> 00:22:27 transunsaturated fats there's this sysunsaturated fat also. 247 00:22:27 --> 00:22:30 And if you look here, you can see that these side chains 248 00:22:30 --> 00:22:34 are kinked. And the reason they're kinked is that the hydrogens across 249 00:22:34 --> 00:22:38 the double bond are on the same side of the molecule as one another. 250 00:22:38 --> 00:22:42 That has the effect of not allowing the side chains to pack tightly. 251 00:22:42 --> 00:22:46 So you get side chains floating all over the place, 252 00:22:46 --> 00:22:50 and that changes the energy of the fat so that it is a liquid rather 253 00:22:50 --> 00:22:54 than a solid. And these are much better for you, 254 00:22:54 --> 00:22:58 these sysunsaturated fats. And you find them in lots of 255 00:22:58 --> 00:23:03 vegetable oils and vegetable fats. OK. Here's another one. 256 00:23:03 --> 00:23:10 Here's our joke for the day. OK. Steroids. Steroids are a 257 00:23:10 --> 00:23:17 lipid. They are derived from, OK, so let me just say for this 258 00:23:17 --> 00:23:24 unsaturated. I mentioned sys and transunsaturated fats. 259 00:23:24 --> 00:23:32 I'm going to mention steroid hormones. 260 00:23:32 --> 00:23:35 There are a few of them. Steroids are not all hormones, 261 00:23:35 --> 00:23:38 although cholesterol, we don't think of cholesterol as a hormone, 262 00:23:38 --> 00:23:42 although, actually, I think it really is. Vitamin D is a vitamin 263 00:23:42 --> 00:23:45 that works together with other molecules. Cortisol and 264 00:23:45 --> 00:23:49 testosterone are classically thought of as hormones. 265 00:23:49 --> 00:23:52 Very small amounts of them will change body function in profound 266 00:23:52 --> 00:23:56 ways. And we'll have more to say about testosterone later 267 00:23:56 --> 00:24:00 on in the course. The androgens that lead to people 268 00:24:00 --> 00:24:05 who have large green muscles are a class that are related to 269 00:24:05 --> 00:24:11 testosterone, androsterone and so on, but they're all the structures with 270 00:24:11 --> 00:24:16 these four rings in this characteristic way are all 271 00:24:16 --> 00:24:21 characteristic of a steroid. All right. So that is all I have 272 00:24:21 --> 00:24:27 to say about lipids. Let's move onto either number two 273 00:24:27 --> 00:24:34 here, carbohydrates. So carbohydrates have as their -- 274 00:24:34 --> 00:24:45 OK. Carbohydrates. 275 00:24:45 --> 00:24:52 Also classically known as sugars. 276 00:24:52 --> 00:24:57 And because of that there are a couple of functions of carbohydrates. 277 00:24:57 --> 00:25:02 They are the major source of short-term energy. 278 00:25:02 --> 00:25:07 If you need short-term energy you use carbohydrates. 279 00:25:07 --> 00:25:12 They're also a carbon source, so we'll talk extensively about 280 00:25:12 --> 00:25:17 building up molecules. And you need carbons to do that. 281 00:25:17 --> 00:25:22 Carbohydrates serve as a carbon source for building other molecules. 282 00:25:22 --> 00:25:27 And, as I'll mention briefly, they also can convey information. 283 00:25:27 --> 00:25:31 Carbohydrates have, as their chemical principle, 284 00:25:31 --> 00:25:35 so we can find a chemical principle that unifies them, 285 00:25:35 --> 00:25:40 they all contain carbons that are joined to a hydrogen and to a 286 00:25:40 --> 00:25:44 hydroxyl, and obviously to other things as well. 287 00:25:44 --> 00:25:49 So you can loosely give carbohydrates the formula of CH2O to 288 00:25:49 --> 00:25:53 the N. Now, the monomer, now we can actually, with lipids 289 00:25:53 --> 00:25:57 it's very hard to talk about a monomer and a polymer. In 290 00:25:57 --> 00:26:02 carbohydrates one can. And one can talk about the monomer, 291 00:26:02 --> 00:26:08 which is called a monosaccharide which polymerizes to form a 292 00:26:08 --> 00:26:14 polysaccharide. And they do so. 293 00:26:14 --> 00:26:19 One can also get, let me just write it here, 294 00:26:19 --> 00:26:25 one can also get disaccharides where disaccharides are two and 295 00:26:25 --> 00:26:31 polysaccharides are really more than two. 296 00:26:31 --> 00:26:36 They're usually more than ten or so, but for our purposes more than two 297 00:26:36 --> 00:26:42 is fine. OK. So in order to form a polymer of carbohydrates, 298 00:26:42 --> 00:26:48 one forms a glycosidic linkage or glycosidic bond. 299 00:26:48 --> 00:26:54 So it's a covalent bond where one has -- 300 00:26:54 --> 00:27:09 Two carbohydrates or two 301 00:27:09 --> 00:27:17 monosaccharides interacting with one another through a condensation 302 00:27:17 --> 00:27:26 reaction, classical condensation reaction with the elimination of 303 00:27:26 --> 00:27:35 water to give you, excuse me, yeah, no. 304 00:27:35 --> 00:27:40 I wrote this down incorrectly in my notes. Apologies. 305 00:27:40 --> 00:27:46 OK. To give you a glycosidic linkage. OK? So this is a 306 00:27:46 --> 00:27:51 classical condensation reaction. Now, polysaccharides or 307 00:27:51 --> 00:27:57 carbohydrates are interesting. They can either be rings or they 308 00:27:57 --> 00:28:01 can be linear. And they can also be modified. 309 00:28:01 --> 00:28:05 So like the lipids where you could get phospholipids, 310 00:28:05 --> 00:28:09 carbohydrates can have various extra things added onto them that changes 311 00:28:09 --> 00:28:13 their properties. Let's take a look at a few of them. 312 00:28:13 --> 00:28:17 Here are some monosaccharides. And the ones that are super important 313 00:28:17 --> 00:28:21 for you to know are these two down here. OK? And you should know the 314 00:28:21 --> 00:28:25 structures of these. This is ribose and deoxyribose. 315 00:28:25 --> 00:28:29 And the reason for this will become clear when I move onto the next 316 00:28:29 --> 00:28:33 class of macromolecules, DNA and RNA, because those sugars 317 00:28:33 --> 00:28:37 are part of DNA and RNA. OK? So these ribose and deoxyribose 318 00:28:37 --> 00:28:42 are five carbon rings. You can see they're not quite a 319 00:28:42 --> 00:28:47 five, they're not a five carbon ring. They are five carbon sugars. 320 00:28:47 --> 00:28:52 And this is an isomer of one of the forms that this five carbon sugar 321 00:28:52 --> 00:28:57 can take. OK. So some of you may be lactose 322 00:28:57 --> 00:29:02 intolerant and might not be able, in fact, to enjoy digging in and 323 00:29:02 --> 00:29:07 enjoying a bowl of creamy delicious ice cream. 324 00:29:07 --> 00:29:11 So, in fact, the reason, I didn't make that up, though the 325 00:29:11 --> 00:29:15 reason you cannot is because you need a specific protein, 326 00:29:15 --> 00:29:19 a specific enzyme that we'll talk about, not the specific enzyme, 327 00:29:19 --> 00:29:23 but we'll talk about the notion of enzymes in a couple of lectures that 328 00:29:23 --> 00:29:28 breaks a bond between the disaccharide that is lactose. 329 00:29:28 --> 00:29:33 So lactose is a disaccharide that is a galactose joined together with a 330 00:29:33 --> 00:29:38 glucose. OK. So these are six carbon sugars. 331 00:29:38 --> 00:29:43 And there's a bond between the two of them that you have to break in 332 00:29:43 --> 00:29:49 order to digest the lactose from milk. And if you cannot then this 333 00:29:49 --> 00:29:54 lactose causes you digestive issues. Here on the top is sucrose which is 334 00:29:54 --> 00:30:00 a dimer, a disaccharide, a glucose joined to a fructose. 335 00:30:00 --> 00:30:04 OK. So here's an interesting question. I don't know when you 336 00:30:04 --> 00:30:09 were little, but when I was little I used to try and eat grass. 337 00:30:09 --> 00:30:13 And, clearly, that doesn't work very well. Humans cannot eat grass. 338 00:30:13 --> 00:30:18 Whereas, cows and many animals can survive just very well on a diet of 339 00:30:18 --> 00:30:22 just plants. And that is because we cannot deal with this carbohydrate 340 00:30:22 --> 00:30:27 in plants called cellulose. Now, interestingly, 341 00:30:27 --> 00:30:31 we can digest other plant carbohydrates, 342 00:30:31 --> 00:30:36 particularly starch or amylose which is what most of the carbohydrates 343 00:30:36 --> 00:30:41 that we eat come from. The reason for this is interesting, 344 00:30:41 --> 00:30:45 and it has to do with these glycosidic linkages. 345 00:30:45 --> 00:30:50 OK. So here we have a linkage between glucose, 346 00:30:50 --> 00:30:55 two glucoses that give something called maltose. 347 00:30:55 --> 00:30:59 And maltose is part of the starch carbohydrate, or the amylose 348 00:30:59 --> 00:31:04 carbohydrates. And what you should know here about 349 00:31:04 --> 00:31:09 these chains, these circles, and you should remember from old 350 00:31:09 --> 00:31:14 chemistry as if there is a bold part in the front of this ring then it is 351 00:31:14 --> 00:31:18 coming out of the board towards you. OK. So here you have these two 352 00:31:18 --> 00:31:23 glucoses linked such that a bond joining them is coming out of the 353 00:31:23 --> 00:31:28 board towards you. Now, conversely, 354 00:31:28 --> 00:31:33 and this is called an alpha 1, 4 bond where the bonds coming out of 355 00:31:33 --> 00:31:38 the board towards you in cellobiose, which is a precursor for cellulose, 356 00:31:38 --> 00:31:43 the grass carbohydrate, the bond goes back, OK, back away 357 00:31:43 --> 00:31:48 from the rings. And that's called a beta 1, 358 00:31:48 --> 00:31:53 4 linkage. And we can break this alpha 1, 4 linkage but not this beta 359 00:31:53 --> 00:31:59 1, 4 linkage. And I'll come back to this when we talk about enzymes. 360 00:31:59 --> 00:32:03 We do not have the correct enzymes to do so. And finally with respect 361 00:32:03 --> 00:32:07 to carbohydrates, I mentioned that they were sources 362 00:32:07 --> 00:32:11 of information. One of the interesting sources of 363 00:32:11 --> 00:32:15 information from carbohydrates is in your blood cells. 364 00:32:15 --> 00:32:19 So your blood group is as a result of adding specific chains of 365 00:32:19 --> 00:32:23 specific carbohydrates, specific monosaccharides, 366 00:32:23 --> 00:32:27 and a whole bunch of them, onto particular red blood cell 367 00:32:27 --> 00:32:32 surface proteins. So if you're an A blood group, 368 00:32:32 --> 00:32:37 you have a series of specific carbohydrates added on in order to 369 00:32:37 --> 00:32:42 one of these cell surface proteins. If you're a B blood group, you also 370 00:32:42 --> 00:32:46 have this carbohydrate chain but the specific carbohydrates, 371 00:32:46 --> 00:32:51 the specific monosaccharides are different. All right. 372 00:32:51 --> 00:32:56 So let's move onto the reason that you're all really sitting here. 373 00:32:56 --> 00:33:01 So, really, the reason that you're all sitting here is because of this 374 00:33:01 --> 00:33:05 next group of macromolecules. And I don't mean that that's the 375 00:33:05 --> 00:33:09 reason, you know, you're alive, that you exist in life. 376 00:33:09 --> 00:33:12 I mean that's the reason you're talking 7.013. 377 00:33:12 --> 00:33:16 Because nucleic acids are the thing that have, and the understanding of 378 00:33:16 --> 00:33:20 nucleic acids is what has revolutionized biology so that -- 379 00:33:20 --> 00:33:28 So that everyone in every discipline 380 00:33:28 --> 00:33:33 at MIT has some association, has some way of using the 381 00:33:33 --> 00:33:39 information from nucleic acids to their advantage and in their careers. 382 00:33:39 --> 00:33:44 OK. So nucleic acids, as I'll discuss, do serve as an 383 00:33:44 --> 00:33:49 energy source or as a way of storing energy, but most important is their 384 00:33:49 --> 00:33:54 function, and that's a super important function. 385 00:33:54 --> 00:34:00 But the reason you're talking 7. 13 is not because of that. 386 00:34:00 --> 00:34:07 The reason you are is because of the information content that nucleic 387 00:34:07 --> 00:34:14 acids are able to carry and to perpetuate from one generation to 388 00:34:14 --> 00:34:21 another. So, really, what they are involved in is 389 00:34:21 --> 00:34:29 information storage and transfer. All right. 390 00:34:29 --> 00:34:35 So, again, as for carbohydrates, we can talk about monomers and we 391 00:34:35 --> 00:34:41 can talk about polymers. The monomers for nucleic acids is a 392 00:34:41 --> 00:34:47 nucleotide. Oh, OK. Enough air play. 393 00:34:47 --> 00:34:53 Watson and Crick get a lot of air play, so that's enough. 394 00:34:53 --> 00:35:00 You can go and look at their picture at your leisure. OK. 395 00:35:00 --> 00:35:03 So I've taken this from your book and I'm going to draw some stuff on 396 00:35:03 --> 00:35:07 the board. I want to point out that in the new edition of your book, 397 00:35:07 --> 00:35:11 you will find there is a mistake in the structure of your nucleotides. 398 00:35:11 --> 00:35:15 That was true in the last edition. I wrote to the senior author. I 399 00:35:15 --> 00:35:19 said there is a problem with the sugar structure in the nucleic acids. 400 00:35:19 --> 00:35:23 Would you please change it for the next edition? And he said 401 00:35:23 --> 00:35:27 absolutely we will. And the new edition came out and 402 00:35:27 --> 00:35:30 there we are. OK? So we'll have to, 403 00:35:30 --> 00:35:34 so we're going to deal with that, but I think we'll get you all free 404 00:35:34 --> 00:35:38 copies, or your successors free copies of the next edition of the 405 00:35:38 --> 00:35:42 book or something to make up for this. But, actually, 406 00:35:42 --> 00:35:46 it's a good exercise for you because you can learn the proper structure 407 00:35:46 --> 00:35:50 of the sugar. So what is a nucleotide? OK. 408 00:35:50 --> 00:35:54 A nucleotide, as you gather, has a sugar associated with it. 409 00:35:54 --> 00:35:58 It has, the sugar it has associated it with is either ribose 410 00:35:58 --> 00:36:02 or deoxyribose. This sugar is joined to a phosphate 411 00:36:02 --> 00:36:07 group. And the sugar is also joined to something called a base. 412 00:36:07 --> 00:36:12 Now, the sugar is a five carbon ring. I'll come back to this slide 413 00:36:12 --> 00:36:17 in a moment and we'll talk about the bottom part of it. 414 00:36:17 --> 00:36:22 The sugar is a five carbon ring. OK? Here it is. It's actually a 415 00:36:22 --> 00:36:27 four carbon ring with an oxygen and there's another carbon 416 00:36:27 --> 00:36:32 sticking out. And what they've done in your book 417 00:36:32 --> 00:36:36 is to forget about this carbon up here and turn this oxygen here into 418 00:36:36 --> 00:36:40 a carbon. So it's kind of weird. Later in the book the structure is 419 00:36:40 --> 00:36:44 correct, but you should bear this in mind. I've pointed this out in your 420 00:36:44 --> 00:36:49 PowerPoint handouts. OK. So what's really important, 421 00:36:49 --> 00:36:53 there are two things about nucleotides that are really 422 00:36:53 --> 00:36:57 important. One is this phosphate and sugar because that is the way 423 00:36:57 --> 00:37:02 nucleotides polymerize with one another. 424 00:37:02 --> 00:37:07 And the other is the base which is the part of the molecule that gives 425 00:37:07 --> 00:37:13 rise to the information. Actually, let me just stop this and 426 00:37:13 --> 00:37:18 write the bases here. So let me tell you the names of the 427 00:37:18 --> 00:37:24 bases. And you need to know these. There are four of them. Five 428 00:37:24 --> 00:37:30 really. There is adenine, 429 00:37:30 --> 00:37:37 guanine, cytosine and thymine. And then there's also something 430 00:37:37 --> 00:37:45 that's an alternate to thymine called uracil. 431 00:37:45 --> 00:37:52 And they are abbreviated by their first letters. 432 00:37:52 --> 00:38:00 OK? So A, G, C and T and U. And you need to know these letters. 433 00:38:00 --> 00:38:07 And you need to know what they stand for. All right. 434 00:38:07 --> 00:38:16 So let me, in the last two minutes, 435 00:38:16 --> 00:38:22 tell you about some properties. Actually, let me tell you about the 436 00:38:22 --> 00:38:28 bond that forms the nucleic acid polymer. And then I'll finish this 437 00:38:28 --> 00:38:34 off next lecture. The bond that forms the polymer is 438 00:38:34 --> 00:38:44 called a phosphodiester bond. 439 00:38:44 --> 00:38:48 And it forms by the reaction of the phosphate and the sugar with one 440 00:38:48 --> 00:38:53 another, or groups on the phosphate and the sugar with one another. 441 00:38:53 --> 00:38:58 The base has got nothing to do with this polymerization. 442 00:38:58 --> 00:39:03 So we have here a phosphate with some oxygens that's joined onto the 443 00:39:03 --> 00:39:09 sugar. So I'm abbreviating the phosphate P, I'm abbreviating the 444 00:39:09 --> 00:39:15 sugar S, and I'm abbreviating the base B. OK? And the sugar is 445 00:39:15 --> 00:39:21 attached to a base. And the sugar has, in one part of 446 00:39:21 --> 00:39:27 it, a reactive hydroxyl group. And the phosphate has some reactive 447 00:39:27 --> 00:39:31 oxygen groups. And let's not worry too much about 448 00:39:31 --> 00:39:35 bonds and double bonds and things for now. 449 00:39:35 --> 00:39:51 OK. And I'm not worrying about 450 00:39:51 --> 00:39:55 charges on oxygens and things either. What happens, 451 00:39:55 --> 00:39:59 basically, is that there is an interaction between this hydroxyl 452 00:39:59 --> 00:40:03 group and the phosphate group, OK, or one of the oxygens on the 453 00:40:03 --> 00:40:07 phosphate group. And you get out of this a linkage 454 00:40:07 --> 00:40:26 that goes -- 455 00:40:26 --> 00:40:31 OK. Where you get between the sugar and the phosphate this 456 00:40:31 --> 00:40:37 phosphodiester linkage. OK? An OPO linkage that joins the 457 00:40:37 --> 00:40:42 two sugars to one another through a phosphate molecule. 458 00:40:42 --> 00:40:48 This is your phosphodiester bond. And I want to point out that the 459 00:40:48 --> 00:40:53 bases, which I will tell you next time, are the part of the molecule 460 00:40:53 --> 00:40:59 that carries the information, have got nothing to do with the 461 00:40:59 --> 00:41:04 polymerization of nucleic acids. So I'm going to continue with this 462 00:41:04 --> 00:41:08 next time and briefly finish nucleic acids and then move 463 00:41:08 --> 00:41:11 onto the proteins.